1. Field of the Invention
The present invention relates to ion pumps, devices utilized to maintain vacuum within vacuum envelopes.
2. State of the Art
Problems are frequently encountered in glass-enclosed vacuum chambers in that gases are unavoidably introduced into the vacuum chamber through a number of sources. Such sources may include helium diffusion through the glass and typical "outgassing" of gases from materials located within the envelope due to electron stimulation, heat or the passage of time. These sources can introduce gases into the vacuum chamber and can lead to an increase in the chamber's pressure (a reduction of the vacuum).
Helium diffusion and the resulting pressure increase is especially a problem where the volume of the evacuated chamber is small compared to the surface area of the enclosing glass envelope. This situation occurs in planar vacuum chambers such as those of panel display devices. For example, an envelope formed by two 127 mm by 177.8 mm (5 inch by 7 inch) rectangular pieces of glass enclosing a chamber 1.5 mm thick would have a surface area of approximately 40,000 square mm. The corresponding volume of the chamber would be 34,000 cubic mm, a volume-to-area ratio of less than one.
In such a volume, unaided by any means for maintaining the chamber's vacuum, the partial pressure of Helium would rise within the evacuated chamber until it reached equilibrium with the level of Helium in the outside atmosphere. Helium naturally diffuses through glass, and under normal conditions, the diffusion would raise the level of helium within the chamber to the level of helium in atmosphere, resulting in an increase in pressure in the chamber to approximately 10 E-3 Torr. In an application where it is important to maintain a high vacuum, for example 10 E-6 Torr, some form of vacuum pumping becomes necessary.
Additionally, where an electrical field is present within the envelope, a dynamic environment will result which accelerates the continuous outgassing of materials located within the chamber. The outgassing is the result of the presence of electron bombardment within the chamber and is referred to as electron stimulated desorption. desorption of gases can also occur as a result of heat generated by various phenomena, and by the mere passage of time. Gas desorption will further reduce the vacuum unless some means is employed to continuously evacuate the envelope.
In the past, it has not been practicable to provide a continuous vacuum outside of a laboratory or a manufacturing environment. While passive gettering materials can be utilized within the vacuum chamber, they are not able to getter inert gases, such as helium, nor are they able to getter organic gases desorbed into the vacuum chamber. Various vacuum pumps are utilized in order to maintain vacuum against not only active materials, but also inert and organic materials. Previous vacuum pump technology consists of relatively large devices (for example, large turbo-pumps and cryo-pumps to the somewhat smaller cross-field ion pumps) which can be attached to an evacuated chamber via a port permitting access between the pump and the chamber. These pumps have been utilized in maintaining vacuum for various microwave and x-ray tubes in manufacturing and laboratory environments.
However, it has not been possible with previous vacuum pump technology to provide for continuous active maintenance of a small vacuum envelope while allowing for portability of the device incorporating the vacuum envelope, and retaining the device's original dimensions. The present invention addresses these needs.